1. Field of the Invention
The present invention relates to an antiglare film, an antireflection film, a polarizing plate and an image display device.
2. Description of the Related Art
In various image display devices such as liquid crystal display device (LCD), plasma display panel (PDP), electroluminescent display (ELD) and cathode ray tube display device (CRT), an antiglare film or an antiglare antireflection film is used on the display surface so as to prevent reduction in the contrast due to reflection of outside light or disturbing reflection of an image. With widespread usage of the image display device in an office or household environment, it is required to enhance the antiglare property of preventing a fluorescent lamp in room or the viewer's figure from causing disturbing reflection on the display surface and more improve the display contrast in a bright place.
In an antiglare film, a light-transmitting particle is added to the antiglare layer so as to utilize an antiglare function of causing light scattering by making use of unevenness produced on the antiglare layer surface (surface scattering property) and a light scattering function owing to difference in the refractive index between the light-transmitting particle and the light-transmitting resin in the antiglare layer (internal scattering property). The surface scattering property can be obtained by controlling the surface unevenness by the utilization of aggregating property of the light-transmitting particle.
One of means for improving the dispersibility of the light-transmitting particle is addition of a dispersant. There is disclosed a coating material composition for the formation of a hardcoat layer, where a (meth)acrylate copolymerization product containing an amino group or a quaternary ammonium group is contained in the fine particle (JP-A-2004-217847, the term “JP-A” as used herein meaning an “unexamined published Japanese patent application”). However, JP-A-2004-217847 limits the fine particle to a silica fine particle and is silent on a resin particle.
It is well known in the art that in the case of a resin particle of less than 6 μm, the aggregating property of the particle can be controlled by changing its composition, but in the case of a light-transmitting resin particle having an average particle diameter of 6 to 20 μm, even when the composition of the particle is changed, aggregation of particles cannot be prevented. The reason therefor is considered because the large particle diameter allows for a high precipitation rate to bring about thickening by drying immediately after the coating and this requires the particle to travel a long precipitation distance until it becomes non-flowable.
That is, aggregation of particles proceeds by a snowball effect while repeating a cycle of a process where the collision frequency of particles with each other is increased and the apparent particle diameter of the particle resulting from aggregation by collision becomes large to make the precipitation rate higher and the precipitation distance longer and further increase the collision frequency of particles with each other. Also, in the case where the aggregating property of particle is large, the arithmetic mean roughness of the film is increased and denseness of black in a bright place is deteriorated to impair the image quality. Thus, it is demanded to reduce the aggregating property of the particle.